This invention is directed to blow molding systems that employ previously formed parisons or preforms that are reheated in a reheating stage, typically from an ambient temperature to a molding temperature, prior to blow molding. More particularly, the present invention is directed to an additional preheater for such a blow molding system for preconditioning the preforms prior to the reheating stage so that a temperature differential is achieved in each preform that allows for a preferential distribution of the resin forming the preform during blow molding.
In a blow molding operation employing previously formed parisons, the temperature of each preform as it is being blown is one of the most critical process variables. Ideally, the temperature should be as consistent as possible from one preform to the next in order to achieve product uniformity of the blow molded product. The rate at which a preform can be heated from ambient temperature to a temperature suitable for blow molding is controlled in part by the thermal conductivity of the resin forming the preform. Other factors include the thickness of the material of the preform, the distance between the preform and the source of heat, the energy density or power of the heat source, and the need to not waste heat. Some prior art processes have adopted reheating profiles that are intended to achieve the desired reheating in a particularly expeditious manner such as that disclosed in U.S. Pat. No. 5,066,222.
Most reheating processes include some cooling or at least reduced heating periods to allow for equilibration of the temperature through the wall thickness of the preform. The time typically required to reheat a preform from ambient temperature to blow molding temperature far exceeds the time required to perform the blow molding and cooling steps. Thus, the time required to achieve the desired temperature characteristics for the preform, within the limits of the prior art reheating and equilibrating steps, when coupled with the physical geometry of the reheating apparatus, places a practical limit on the maximum line speed for the blow molding operation for each given apparatus. Any increases in line speed must be compensated by correspondingly lengthening the reheating apparatus so that the residence time of any preform in the reheating process is sufficient to achieve the necessary temperature profile prior to blow molding.
On occasion, it is desirable to achieve some temperature variation within each preform so that some desirable physical property can be obtained in the blow molded product. Such temperature variations have been achieved in the reheating process. Examples of the creation of temperature variations in preforms can be found in U.S. Pat. Nos. 3,775,524; 3,950,459; 4,079,104; 4,117,050; 4,423,312; 5,292,243; 5,681,521; 5,853,775 and 6,146,134. Generally, the temperature variation required to achieve any significant property difference during a blow molding process or in the product obtained from such a process is at least about 3° C. It has been recognized in U.S. Pat. No. 5,607,706 and some of the prior art discussed therein, that variations in temperature that are present in preforms prior to a reheating process can affect the final temperature of the preform and can affect the subsequent blow molding process. However, it has not previously been recognized that a purposeful introduction of temperature variations into preforms prior to a normal reheating process can be used advantageously to create desirable temperature variations affecting the blow molding process.